Setting the word wrap to a specific width applies only in plain text messages. The adjustment is in the config editor. '''Tools|Options|Advanced|General→Config Editor''' and search for ''wrap''. You'll see one option with a numeric value that you can change, and you'll see some other settings about whether or not to wrap text. I'll have to leave you to experiment with these as I don't understand them. :-( If you're using HTML then it's customary for text to flow and be re-wrapped automatically to fit the display window. Plain text can also be set to flow to the window width (known as "format=flowed"), though some users like to point out that this isn't implemented well in Thunderbird and becomes untidy when messages are re-wrapped when quoted in replies. If you feel that email should wrap at a fixed width then I guess you're most at home with plain text messages, so I'd urge you to check the account settings, under Composition, and make sure the "Compose messages in HTML format" checkbox is unticked.